1. Field of the Invention
The present invention relates to carbon fibers, and more particularly, to a method of producing short carbon fibers by a vapor phase growth process (hereinafter referred to as vapor growth carbon fibers).
2. Description of the Related Art
Carbon fibers have been produced by preparing PAN (polyacrylonitrile) fibers, pitch fibers or the like as a precursor and stabilizing and graphitizing the fibers, and have been used in composite products of the aerospace industry, sports goods industry, leisure goods industry and the like, because these carbon fibers have superior properties such as a light weight, high tensile strength and high modulus. If the carbon fibers can be produced at a cost lower than currently required, it is expected that these carbon fibers will be widely used in the automobile industry and in general industry or the like. Although vapor growth carbon fibers are discontinuous fibers (short fibers), the fibers are directly formed by thermally decomposing gaseous hydrocarbons, such as benzene and methane, and utilizing a catalyst of transition metal fine particles, so that the production cost thereof can be greatly reduced compared with that of the above mentioned carbon fibers, and the produced fibers will have superior physical and chemical properties.
Conventional methods of producing vapor growth carbon fibers are classified, from a standpoint of the existing state of transition metal particle catalysts, into (a) a substrate seed method using a substrate carrying the catalyst thereon (cf. Japanese Examined Patent Publication (Kokoku) No. 58-22571 and Japanese Unexamined Patent Publication (Kokai) No. 57-117622) and (b) a space seed method in which the catalyst is directly fed into a reaction space (cf. Japanese Examined Patent Publication No. 62-242). In the substrate seed method a sufficient residence time can be allowed to obtain grown fibers having a diameter of several micrometers, but this method has a low utilization of the reaction space due to the use of the substrate, which is a weak point for use thereof on an industrial scale. The space seed method provides an increased utilization of the reaction space by using the reaction space three-dimensionally, but has the following problems from the aspect of the use thereof on an industrial scale.
A. The generation and growth of the vapor growth carbon fibers requires a large amount of heat for preheating a carrier gas and gases used, preheating and thermally discomposing hydrocarbon, and maintaining a reaction (growing) temperature, and thus a large amount of heat must be fed to the reaction space from outside. Although a feed density of heat to the space is increased, a convection heat transfer from a wall of a reaction tube can not provide a sufficiently heat flux, but the production of the vapor growth carbon fibers depends upon the efficiency of the convection heat transfer. At a vapor growth temperature of from 950.degree. to 1300.degree. C., a radiation heat transfer is more effective than a convection heat transfer, but a carrier gas such as hydrogen and argon and a raw material (hydrocarbon) gas such as benzene and methane used for the vapor growth of the carbon fibers are almost transparent to the radiation, and thus the radiation heat transfer from the wall of the reaction tube has little effect.
B. The carbon fibers grow in random directions in the reaction space, with the result that the fibers are intermingled or aggregated to form flossy masses having a very low bulk density. These flossy masses (products) come in various sizes, and therefore, have different resistances to gas flow transportation. Furthermore, some of these flossy masses adhere to the wall of the reactor tube, due to carbon fibers growing out of the wall, and thus these adhered flossy masses have a large resistance to the gas flow transportation. As the result, the differences of the resistances of the flossy products to the gas flow transportation does not allow a practical control of the residence time (growing time) of the carbon fibers, which prevents the use of remarkably profitable features in industrial production using a space seed method, such as a continuous feed of a gaseous raw material and a continuous discharge of vapor growth carbon fibers by the gas flow transportation. Accordingly reaction control is greatly hindered, and thus improvements to the reactor and/or operation thereof are required.
C. The above-mentioned flossy masses (products) are hard to handle during the following steps of transportation, storage, composition with resin or the like, and thus it is necessary to decompose or take each of the flossy products apart.
An improved substrate seed method has been proposed in which narrow and long small substrates are prepared and are set in the reaction tube, to increase the utilization of the reaction space (cf. Japanese Unexamined Patent Publication No. 59-223299). This proposed method is advantageous in that the space utilization is increased and a continuity of the substrate seed method is obtained, but it is still necessary to insert to and take out the substrates from the reaction tube, to feed raw materials, and to discharge the products (vapor growth carbon fibers).